0.1 n Sodium thiosulfate Preparation and Standardization

Take 25 g of sodium thiosulfate pentahydrate (Na2S2O3·5H2O) in 500 mL of freshly boiled and cooled water, and add 0.11 g of sodium carbonate (Na2CO3).Dilute to 1 L with freshly boiled and cooled water, and let stand for 24 h. Store the solution in a tightly closed glass bottle.

Standardization

Take 1 g of primary standard potassium dichromate (K2Cr2O7), transfer to a platinum dish, and dry at 120°C for 4 h. Cool in a desiccator.Weigh accurately 0.21g of the dried K2Cr2O7, and transfer to a 500-mL glass-stoppered conical flask. Add 100 mL of water, swirl to dissolve, remove the stopper, and quickly add 3 g of potassium iodide (KI), 2 g of sodium bicarbonate (NaHCO3), and 5 mL of hydrochloric acid (HCl). Stopper the flask quickly, swirl to ensure mixing,and let stand in the dark for 10 min. Rinse the stopper and inner walls of the flask with water and titrate with the Na2S2O3 solution until the solution is yellowish green. Add 2 mL of starch solution (10 g/L), and continue the titration to the disappearance of the blue color.